1. Field of the Invention
The present invention relates to a method used in a wireless communication system and related communication device, and more particularly, to a method of handling an enhanced physical downlink control channel (EPDCCH) and related communication device.
2. Description of the Prior Art
A long-term evolution (LTE) system supporting the 3rd Generation Partnership Project (3GPP) Rel-8 standard and/or the 3GPP Rel-9 standard are developed by the 3GPP as a successor of a universal mobile telecommunications system (UMTS), for further enhancing performance of the UMTS to satisfy increasing needs of users. The LTE system includes a new radio interface and a new radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, a radio access network known as an evolved universal terrestrial radio access network (E-UTRAN) includes multiple evolved Node-Bs (eNBs) for communicating with multiple user equipments (UEs), and communicating with a core network including a mobility management entity (MME), a serving gateway, etc., for Non-Access Stratum (NAS) control.
A LTE-advanced (LTE-A) system, as its name implies, is an evolution of the LTE system. The LTE-A system targets faster switching between power states, improves performance at the coverage edge of an eNB, and includes advanced techniques, such as carrier aggregation (CA), coordinated multipoint (COMP) transmission/reception, UL multiple-input multiple-output (MIMO), etc. For a UE and an eNB to communicate with each other in the LTE-A system, the UE and the eNB must support standards developed for the LTE-A system, such as the 3GPP Rel-10, Rel-11, or later versions.
A physical downlink control channel (PDCCH) (i.e., legacy PDCCH) is used for carrying control information in a wireless communication system conforming to the 3GPP Rel-8, Rel-9 and/or Rel-10 standards. As the 3GPP standard is improved, the PDCCH is not suitable for advanced network deployments and transmission schemes such as heterogeneous networks and the CoMP due to its inability of frequency domain inter-cell interference cancellation (FDM-ICIC), lack of finer granularity and insufficient capacity. In 3GPP Rel-11 standard, an enhanced physical downlink control channel (EPDCCH) is proposed to carry the control information under various scenarios flexibly and robustly. It is agreed that the number of enhanced control channel elements (ECCEs) for the EPDCCH in a physical resource block (PRB) is fixed. The number of the ECCEs in the PRB can be 2 or 4, and may depend on a length of the cyclic prefix (CP) and a type of the subframe.
It is noted that the basic unit for the EPDCCH transmission is the ECCE. The ECCEs can be aggregated for an EPDCCH transmission. The number of ECCEs aggregated is called an aggregation level. The aggregation level is usually a power of 2, and EPDCCH aggregation level could be 1, 2, 4, 8, 16 and 32 ECCEs. However, the number of available resource elements (REs) in an ECCE varies a lot in different cases, and may depend on the presence of other signals such as a cell-specific reference signal (CRS) and/or the PDCCH. As a result, it is difficult for the eNB to use the ECCEs efficiently. For example, resources of the EPDCCH may be overused for transmitting control information when one ECCE contains more resource elements and it is enough using one ECCE for most cases, and the resources are wasted. Otherwise, resources of the EPDCCH may be underused for transmitting the control information when one ECCE contains less resource elements, and the eNB is forced to use much more ECCEs. Therefore, the blocking probability may be high. System capacity is limited if resources of the EPDCCH are not used efficiently.
Thus, transmitting the control information via the EPDCCH efficiently while maintaining a low blocking probability and efficient resource usage are an important problem to be solved.